Page:The story of the comets.djvu/145

IX. method of determining by geometrical construction the visible portion of the path of the comet, and invited astronomers to apply these principles to the comets on record, or some of them. He considered that it was very probable that some comets might move in elongated ellipses which near perihelion would scarcely be distinguishable from parabolas; and he even thought that the recent Comet of 1680 might be moving in an ellipse the circuit of which would occupy about 575 years.



Halley (to whose exertions the publication of the Principia was in great measure due, for he bore the labour and expense of its publication) also took this view. Although we now know that the period of that comet is measured by thousands of years Halley's investigations were not without good fruit, for they may be said to have drawn him into a systematic